Multi-zone illuminator with embedded process control sensors

ABSTRACT

A multi-zone illuminator for processing semiconductor wafers is described which comprises a plurality of source lamps and dummy lamps embedded in the reflector side of a lamp housing. The source lamps are arranged in a plurality of concentric circular zones. The illuminator also comprises plurality of light pipes for receiving multi-point temperature sensors to measure the semiconductor wafer temperature and its distribution uniformity. A gold-plated reflector plate is attached to the bottom side of the lamp housing for reflecting and directing optical energy toward the wafer surface. The distance between the reflector plate and the wafer and the lamps and the wafer may be adjusted with the use of a spacial elevator and adaptor assembly. The multi-zone illuminator allows uniform wafer heating during both transient and steady-state wafer heating cycles.

This invention was made with U.S. government support under contractnumber F33615-88-C-5448 (Program Name MMST) awarded by the United StatesAir Force. The U.S. government may have certain rights in thisinvention.

This is a continuation of application Ser. No. 07/870,446, filed Apr.16, 1992, now U.S. Pat. No. 5,268,989.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following co-assigned patent applications are hereby

    ______________________________________                                        Serial No.    Filing Date                                                                             TI Case No.                                           ______________________________________                                        690,426       4/14/91   TI-15255                                              702,646       5/17/91   TI-15188                                              702,792       5/17/91   TI-15844                                              785,386       10/30/91  TI-15734                                              702,798       05/17/91  TI-15843                                              ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to semiconductor device processing andmore particularly to a multi-zone illuminator with embedded real-timeprocess control sensors for uniform wafer processing in rapid thermalprocessing reactors.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with single-wafer rapid thermal processing ofsemiconductors wafers, as an example.

Single-wafer rapid thermal processing (RTP) of semiconductors is apowerful and versatile technique for fabrication ofvery-large-scale-integrated (VLSI) and ultra-large-scale-integrated(ULSI) electronic devices. It combines low thermal mass photon-assistedrapid wafer heating with inert or reactive ambient semiconductor waferprocessing. Both the wafer temperature and the process environment canbe quickly changed and, as a result, each fabrication step can beindependently optimized in order to improve the overall electricalperformance of the fabricated devices.

Rapid thermal processing (RTP) of semiconductor wafers provides acapability for improved wafer-to-wafer process repeatability in asingle-wafer lamp-heated thermal processing reactor. In prior art RTPsystems, equipment manufacturers have spent significant design resourcesto provide uniform wafer heating during the steady-state heating,conditions. These prior art systems are designed with illuminators whichprovide single-zone or very limited asymmetrical multi-zone controlcapability. Thus, with an increase or decrease of power to theilluminator, the entire wafer temperature distribution is affected. As aresult, there are insufficient real-time adjustment and controlcapabilities to adjust or optimize wafer temperature uniformity duringthe steady-state and dynamic transient heat-up and cool-down cycles. Asa result, the transient heat-up or cool-down process segments canproduce slip dislocations as well as process non-uniformities. Variousprocess parameters can influence and degrade the RTP uniformity. Priorart RTP systems are optimized to provide steady-state temperatureuniformity at a fixed pressure. Thus a change in pressure or gas flowrates may also degrade the RTP uniformity.

SUMMARY OF THE INVENTION

Generally, and in one form of the invention, a multi-zone illuminatorused in processing of semiconductor wafers is described which comprisesa plurality of individual source lamps embedded in the reflector side ofa lamp housing. The source lamps are arranged in a plurality ofconcentric circular zones for generating and directing optical energy.The illuminator also comprises a light interference elimination circuit(LIEC) having a plurality of dummy lamps not used for actual waferheating. There is at least one dummy lamp for each circular zone whichcontains a light pipe for receiving a fiber-optic radiance sensor. Thisfiber-optic radiance sensor measures the radiation from the dummy lamps.It is also possible to use a single dummy lamp for all zones. However,the use of one dedicated dummy lamp for each zone is preferred. Agold-plated reflector plate is attached to the bottom side of the lamphousing for reflecting and directing optical energy. The distancebetween the reflector plate and the wafer and the lamps and the wafermay be adjusted with the use of an adjustable elevator (forlamp-to-wafer spacing and an adjustable adaptor between the lamp housingand the process chamber.

An advantage of the invention is uniform wafer heating over a wide rangeof gas pressures and flow rates using the concentric multi-zoneconfiguration.

A further advantage of the invention is the use of adjustablereflector-to-wafer and lamp-to-wafer spacings.

A further advantage of the invention is providing the means forimplementation of multi-point temperature sensors and light interferenceelimination components with minimum space and packaging complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of a single-wafer rapid thermal processingreactor for processing semiconductor devices using the presentinvention;

FIG. 2 is a partially cut-away schematic diagram of a preferredembodiment of the present invention in association with the processingchamber of FIG. 1;

FIG. 3 shows a simplified view of a preferred embodiment of themulti-zone illuminator of the present invention;

FIG. 4 is a schematic diagram of a front view of a multi-zoneilluminator in accordance with the present invention;

FIG. 5 is a schematic diagram of a bottom view of a multi-zoneilluminator according to the present invention;

FIG. 6 is schematic diagram of a side view of a multi-zone illuminatoraccording to the present invention;

FIG. 7a-d are a schematic diagram of the power distribution systemaccording to the present invention;

FIG. 8 is a block diagram showing the operation of the lightinterference elimination circuit (LIEC) in accordance with the presentinvention.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The continuing down scaling of device dimensions in VLSI/ULSI circuitsplaces increasingly challenging demands on the manufacturing tools andtechnologies required to manufacture complex microelectronic devices.Rapid technology advancements have reduced the minimum feature sizes ofdigital integrated circuits into the submicron regime. As a result,short-time and or activated low temperature processes are considered tobe essential for minimizing the dopant distribution problems, increasingthe device fabrication yield, and achieving enhanced process controlduring the device fabrication sequence.

RTP operates based on the single-wafer processing methodology which isconsidered desirable for flexible fast turn-around integrated circuitmanufacturing.

FIG. 1 is a schematic representation of a semiconductor processingreactor 100 that establishes the environment of the present invention.Within a single-wafer RTP reactor 100 such as the Texas Instruments'advanced automated vacuum processor (AVP), may reside semiconductorwafer 60. Beginning at the may reside semiconductor wafer 60. Beginningat the bottom right-hand corner of FIG. 1, gas distribution network 102may comprise two gas manifolds: a non-plasma process gas manifold and aplasma manifold. Non-plasma gas manifold penetrates through reactor wall108 via gas line 104 and process chamber wall 110 to proceed throughground electrode 112 and into gas injector 114. Plasma manifold connectsinto discharge cavity 118 via gas line 106 for generating processplasma. Process plasma activated species pass within plasma dischargetube 120 through reactor casing 108 and process chamber wall 110,through ground electrode 112 and into plasma injector 122 near nonplasmagas injector assembly 114. Above gas injector assembly 114 and supportedby low thermal mass pins 124 appears semiconductor wafer 60. Low thermalmass pins 124 are supported by ground electrode 112 (or a liner, notshown) within process chamber 126. Wafer processing may be conductedwith only thermal activation or with a combination of thermal and plasmaprocess activation.

Process chamber 126 includes optical quartz window 128 which separatessemiconductor wafer 60 from multi-zone illuminator 130 of the presentinvention. In association with multi-zone illuminator 130 may bemulti-point temperature sensor 132 (not shown) as described in U.S. Pat.application Ser. No. 702,646 by M. Moslehi, et al. filed on Apr. 24,1991 and assigned to Texas Instruments Incorporated. Vacuum pumpconnection 134 removes flowing process gas and plasma from processchamber 126 and into pumping package 136. Additionally, isolation gate138 permits passage of semiconductor wafer 60 from vacuum load-lockchamber 140 into RTP process chamber 126. To permit movement ofsemiconductor wafer 60 into process chamber 126, chamber collar liftmechanism 142 supports process chamber collar 110.

Within vacuum load-lock chamber 140 appears cassette 144 ofsemiconductor wafers 60 from which wafer handling robot 146 removes asingle semiconductor wafer 60 for processing. To maintain load-lockchamber 140 under vacuum, load-lock chamber 140 also includes vacuumpump connection 148.

Process control computer 150 controls the processing of semiconductorwafer 60 in RTP reactor 100. Control signals from process controlcomputer 150 include signals to multi-zone controller 152. Controller(or multi-zone controller) 152 provides various signals to multi-zonelamp module power supply 154. Illuminator power supply 154 responsivelyprovides power settings to multi-zone illuminator 130.

Process control computer 150 also directs pressure set points to pumpingpackage 136 as well as gas and plasma inlet flow signals to mass-flowcontrollers in the gas distribution network 102. To provide properactivation of plasma species at discharge cavity 118, process controlcomputer 150 provides a control signal to microwave source 156 which, inthe preferred embodiment, operates at a frequency of 2450 MHZ.

Process control computer 150 checks the status of multi-zone illuminator130 via line 158 for diagnosis/prognosis purposes and provides multipletemperature control signals to multi-zone controller 152 in response totemperature readings of multi-point sensors (not shown). The multi-zonecontroller receives measured multi-point temperature sensor outputs (notshown) as well as the desired wafer temperature setpoint (from computer)and delivers power setpoints to the lamp zone power supplies. Sensinglines 159 between process control computer 150 and multi-zoneilluminator 130 of the present invention include signals frommulti-point temperature sensor (not shown) for real-time semiconductorwafer 60 temperature measurements as well as the status of the zonelamps to monitor aging and failure of the lamps.

FIG. 2 shows a perspective view of the Texas Instruments AVP oradvanced/automated vacuum processor operating as an RTP reactor 100 forpurposes of the present invention. Process chamber 126 is mounted onreactor frame 136. Process chamber 126 rigidly supports multi-zoneilluminator 130. Adjacent to process chamber 126 is vacuum load-lockchamber 140 within which appears cassette 160 for holding semiconductorwafers 144. Adjacent to vacuum load-lock chamber 140 is process controlcomputer 150 which controls the operation of the various elementsassociated processing reactor 100.

FIG. 3 shows a schematic view of the main components of a multi-zoneilluminator 130 of this invention (components shown separate from eachother). The main components include a water-cooled lamphousing/reflector 200, an array of multi-zone heating lamps 328, a lampsocket base plate 322, and a power wiring module 320. The power wiringmodule 320 provides electrical connections between the lamp zones andexternal electrical power supplies (not shown). The base plate 322 holdsall the lamp sockets. The multi-zone heating lamps 328 penetrate throughthe water-cooled lamp tubes 329 within the water-cooled lamphousing/reflector assembly 200. Additional water-cooled tubes 326 with asmaller diameter are included for insertion of a multi-point temperaturesensor system for multi-zone lamp and temperature control. These sensorsare inserted into the multi-zone illuminator assembly 130 throughvarious holes 325, 324, 210, which are aligned between the power wiringmodule 320, lamp base plate 322, and water-cooled lamp housing/reflector200.

The preferred embodiment of the invention is shown more in detail inFIG. 4. The multi-zone illuminator 130 consists of a lamp housing 200which has a series of open spaces or tubes through which an array ofheating lamps 220 and dummy lamps 222 protrude. Dummy lamps 222 areshown in the periphery of housing 200 but they may be located elsewhere.On the bottom of housing 200 is a gold-plated reflector plate 230. Lamphousing 200 is water-cooled to prevent heating of the reflector 230 andthe pyrometer light pipes 210. Referring to FIG. 5, the lamps 220 arearranged vertically as point sources along the axis of the illuminator,distributed over several concentric rings. The preferred embodiment usesfour concentric rings, wherein each ring forms one heating zone. Thenumber of rings (or zones) may be more or less than four. Zone 1consists of 5 heating lamps 240 and a peripheral dummy lamp 242 locatedon the periphery of housing 200. Zone 2 consists of 11 heating lamps 250and a dummy lamp 252 located on the periphery of housing 200. Zone 3consists of 20 lamps 260 and four dummy lamps 262, 264, 266, and 268located on the periphery of housing 200. Zone 4 consists of 29 lamps 270and a dummy lamp 272 located on the periphery of housing 200. In thepreferred embodiment lamps 220 are each 1 KW tungsten-halogen lamps.However, it should be noted that lamp ratings of 500 watts, 750 watts, 2kw or even more may also be used. Other types of lamps different fromthe tungsten-halogen type may also be used.

Referring to FIG. 4, housing 200 is located above and separate fromwafer 60 by optical window 290. Optical window 290 may be made of quartzor another transparent material. On the peripheral bottom edge ofoptical window 290 is a reflective film coating 295. The reflective film295 may be of, for example chromium and may be used to prevent directexposure of the vacuum O-ring seals to lamp light. The distance betweenreflector plate 230 and wafer 60 and the distance between heating lamps220 and wafer 60 is adjusted by adjustable elevator 300 and an adaptorring placed between the lamp housing 200 and the quartz window 290.Motor 350 drives lead screw 330 which itself drives (raises or lowers)nut 370. Nut 370 is connected to carriage 202. Carriage 202 is connectedto wiring module 320 and lamp support. This mechanism raises or lowersthe lamp array with respect to the main lamp housing 200 (reflector230). A separate mechanism (not shown) is provided to adjust the spacingbetween the lamp reflector 230 and the quartz window 290. The elevatormechanism 300 can be used for two purposes: (i) to adjust the relativespacing between the lamp array and the quartz window 290 (with a givenreflector-to-quartz spacing); and (ii) to raise the entire lamp arrayand associated wiring module 320 out of the main lamp housing 200(water-cooled reflector 230). The latter will allow rotating the lamparray in order to replace lamps. As a result, the overall optical fluxof the illuminator and its distribution pattern can be optimized for awide range of process parameters, including chamber pressure and totalgas flow rate. FIG. 4 shows lamp housing 200 at a typical position nearoptical window 290. FIG. 6, shows illuminator 130 from a side view,indicating the radial positions of various lamps in four concentriczones, as well as the pivot for rotation of the illuminator assembly formaintenance.

Power supplies 340 through 370 are three phase power supplies. Referringto FIG. 7, Zone 1 is powered by power supply 340. Lamps 240 may beconnected as follows: Lamps 240a and 240b connected in series betweenphase 1 and phase 2, lamps 240c and 240d connected in series betweenphase 2 and phase 3, lamp 240e and dummy lamp 242 connected in seriesbetween phase 1 and phase 3.

Referring to FIG. 7, zone 2 is powered by power supply 350. Betweenphases 1 and 2 lamps 250a and 250b are connected in series, and lamps250c and 250d are connected in series. Between phases 2 and 3 lamps 250eand 250f are connected in series and lamps 250g and 250h are connectedin series. Between phases 1 and 3 lamps 250i and 250j are connected inseries, and lamp 250k and dummy lamp 252 are connected in series.

Referring to FIG. 7, zone 3 is powered by power supply 360. Betweenphases 1 and 2 the following pairs of lamps may be connected in series:260a and 260b, 260c and 260d, 260e and 260f, 260g and dummy lamp 262.Between phases 2 and 3 the following pairs of lamps may be connected inseries: 260h and 260i, 260j and 260k, 2601 and 260m, 260n and dummy lamp264. Between phases 1 and 3 the following pairs of lamps may beconnected in series: 260o and 260p, 260q and 260r, 260s and dummy lamp266, 260t and dummy lamp 268.

Referring to FIG. 7, zone 4 is powered by power supply 370. Betweenphases 1 and 2 the following pairs of lamps may be connected in series:270a and 270b, 270c and 270d, 270e and 270f, 270g and 270h, 270i anddummy lamp 272. Between phases 2 and 3 the following pairs of lamps maybe connected in series: 270j and 270k, 2701 and 270m, 270n and 270o,270p and 270q, 270r and 270s. Between phases 1 and 3 the following pairsof lamps may be connected in series: 270t and 270u, 270v and 270w, 270xand 270y, 270z and 270aa, 270ab and 270ac. The above describedconnection is an example only. As will be apparent to those skilled inthe art, other combinations are possible (depending on the power supplyand lamp voltage ratings).

Lamps 220 may, for example, be tungsten-halogen or plasma arc lamps andare used to heat the semiconductor wafer 60 during processing. Theconcentric rings of lamps 220 in the preferred embodiment provide asmuch as 58 kw of power for RTP. Because of the multiple lamp sources ineach zone and their proximity to one another, each zone provides acontinuous photon radiation ring at the surface of wafer 200 and resultsin uniform wafer heating. Using multiple independent point source lampsin a zone and connecting them to one power supply is significantly lesscomplicated and more economical and practical than providing a singlering shaped lamp.

Dummy lamps 222 are identical to lamps 220 except that they are placedin housing 200 such that their output radiation is isolated from thewafer 60. The isolation is accomplished by blocking the end sections ofthe dummy light pipes with a cap 224 as shown in FIG. 4. The purpose ofdummy lamps 242, 252, 262, 264, 266, 268, and 272 is to measure thelight modulation depth as is described below, for the purpose of precisepyrometry-based temperature measurement.

Multi-point temperature sensors, such as those described in U.S. Pat.application Ser. No. 702,646 filed Apr. 24, 1991, can be used to performreal-time temperature measurement and uniformity control duringtransient and steady-state thermal cycles. However, lamps producingradiance with infrared wavelengths of less than 3.5 microns are desiredbecause the quartz window 290 is transparent at those wavelengths. Atwavelengths of less than 3.5 microns, pyrometry measurements are,however, subject to lamp interference effects which can causesignificant temperature measurement errors and process repeatabilityproblems. Thus, a light interference elimination circuit (LIEC) isdesired.

Each dummy lamp includes a light interference eliminator circuit (LIEC)pyrometer insert light pipe 205 for LIEC modulation depth measurementand control for their respective zones. Radiance pyrometers 206 (shownin FIG. 8) associated with light pipes 205 measure the radiation fromthe dummy lamps 222. Multiple pyrometer insert light pipes 210 are alsoembedded in housing 200 for up to 5 or more radial wafer temperaturemeasurements. Wafer pyrometers 211 associated with light pipes 210measure radiation from both the lamps 220 (due to their interference)and wafer 60. A power modulation source (not shown) is provided formodulating the electrical power source to a selected modulation depthsuch that the output radiation of the dummy lamps 222 varies with theselected AC modulation but the temperature of the wafer 60 remainssubstantially constant. Circuitry is provided for determining thefraction of total radiation collected by pyrometers associated withlight pipes 210 which is emitted by the wafer heating lamps 220 andcalculating the true temperature of wafer 60. Referring now to FIG. 8, asample or portion of the spectral power of the lamp radiation ismeasured by a lamp pyrometer 206. The spectral power includes an ACcomponent ΔI (for reference, ΔI is the peak-to-peak AC signal) and theDC component I. The dummy lamp pyrometers 206 operate in the samespectral band Δλ, and therefore the same center wavelength, as the waferpyrometers. As an example, λ₀ may be 3.3 μm and Δλ may be 0.4 μm.

A lamp pyrometer output signal is provided to a low-pass filter 428which outputs the DC component I. The dummy lamp pyrometer 206 output isalso provided to a high-pass filter 430 and a peak-to-peak to DCconverter 432 which in turn outputs the AC portion of the lamp intensityΔI. (Note: If desired, both blocks 430 and 432 can be bypassed withoutaffecting the functionality of LIEC).

The lamp radiation modulation depth M_(L) is determined by dividing thepeak-to-peak value of the AC component ΔI of the lamp pyrometer 206 byits DC component I in divider 434.

At the same time, the AC component ΔI' of a wafer pyrometer 211 isdetermined from high-pass filter 436 and peak-to-peak to DC converter438. The AC component ΔI' of the wafer pyrometer 211 is then divided bythe measured lamp modulation depth M_(L) in dividing circuit 440. Theoutput Y₂ of the divider is the amount of DC lamp interference, i.e.,the main source of measurement error in the wafer temperature sensor orwafer pyrometer 211. Again, blocks 436 and 438 are eliminated/bypassedif blocks 430 and 432 are eliminated/bypassed.

The lamp interference effect or Y₂ may be subtracted from the DCcomponent I' from the wafer pyrometer 211 (obtained from low-pass filter442) in difference or differential amplifier circuity 444. The output Y₃of the difference circuit 444 is based on the true wafer temperature andsubstantially all lamp interference portion has been eliminated.

This technique also provides real-time data on spectral waferreflectance (or emissivity) in the spectral band of the pyrometer. Thisinformation can be used for real-time correction/compensation of wafertemperature measurement using pyrometry. The spectral reflectance isessentially proportional to the ratio of the AC signal level ΔI'detected by the wafer pyrometer 211 to the AC signal level ΔI detectedby the lamp pyrometer 206. The emissivity is proportional to the outputof divider 452, labeled as Y₄ in FIG. 5. Y₄ is a measure of the waferspectral emissivity at the same center wavelength as the pyrometers. Amore detailed description of the operation of a LIEC (without embeddeddummy lamps) is described in co-pending patent application Ser. No.785,386, filed Oct. 30, 1991 and assigned to Texas Instruments, Inc andis hereby incorporated by reference.

A few preferred embodiments have been described in detail hereinabove.It is to be understood that the scope of the invention also comprehendsembodiments different from those described, yet within the scope of theclaims.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A multi-zone illuminator for processingsemiconductor wafers comprising:a. a lamp housing having a bottom side;b. a plurality of point source lamps embedded in said bottom side, saidlamps arranged in a plurality of concentric circular zones forgenerating and directing optical energy; c. a reflector plate attachedto said bottom side for reflecting and directing optical energy; and d.a means for adjusting the spacing between a wafer and said reflectorplate and the spacing between said wafer and said point source lampswherein the spacing between said wafer and said point source lamps maybe adjusted independently of the spacing between the wafer and thereflector by vertically raising and lowering said point source lamps.